1. Field of the Invention
The invention relates to a switching-type power conversion apparatus, which, for example, may be used for a constant voltage/constant frequency power source apparatus or a power compensation apparatus.
2. Prior Art
With regard to this type of power source, various efforts have been made to improve the apparatus efficiency for the purpose of reducing wasteful consumption of power or better handling generated heat. One example of a effort frequently undertaken to reduce the switching loss of a power semiconductor switching element (e.g., an insulating-gate type bipolar transistor (IGBT)) generally used for this type of power source (the loss occurring at the time at which this type of switching element is turned on or off due to the fact that this type of switching element is not an ideal switch), is to reduce as much as possible the number of switching times. This type of constant voltage power source has also been required to output a constant target voltage, thus requiring a power compensation apparatus to control the output voltage. Thus, this type of power source must be able to change an input voltage to output the target voltage at an output terminal.
FIGS. 3A and 3B illustrate an example of this type of power source, which is disclosed in Japanese Patent Publication No. 2521345 (pp. 3–4, FIG. 1). In the power source shown in FIG. 3A, the serially connected arms V and Y are driven with a pulse synchronized with the inverse of the phase of the input voltage Vin to reduce the frequency with which voltage is switched when compared with a case of a pulse-width modulation (PWM) converter in which switching is performed with a frequency sufficiently higher than a power source frequency, thereby providing a conversion with a high efficiency.
In this case, serially connected arms W and Z are operated as shown in FIG. 3A as a PWM converter switched with a frequency sufficiently higher than a power source frequency that the full bridge converter composed of serially connected arms V and Y and serially connected arms W and Z is operated as an inverter, thereby providing an output having a sinusoidal waveform. Although FIG. 3B schematically shows the serially connected arms W and Z having pulses with a constant interval, in actuality, these pulse widths are increased or decreased depending on a signal waveform subjected to PWM modulation. Then, a DC voltage that is a source of an output voltage is rectified by a full bridge converter by the combination of the input-side arms U and X and the arms V and Y, and thus the DC voltage is equal to or lower than the peak of input voltage.
As a result, this type of converter is limited to providing a step down operation for making the output voltage lower than the input voltage or an operation for providing an input voltage equal to the output voltage. Specifically, there is a limitation that this converter cannot provide a step up operation for making the output voltage higher than the input voltage. The reason that the input side operates as a diode bridge is that the pulse pattern created by the arms V and Y shown in FIG. 3B fires the diode with the timing at which the elements V and Y receive a firing signal. Consequently, the circuit as shown in FIG. 4A can be operated in a similar manner. The output waveforms Vout are also similarly provided in FIG. 3B and FIG. 4B.
FIGS. 5A–5C illustrate another example of this type of power source, which is shown in Japanese Patent Publication No. 3185846 (pp. 5–8, FIG. 2). When the input voltage is relatively close to a voltage to be outputted in the circuit shown in FIG. 5A, a pulse synchronized with the power source as shown in FIG. 5B is used to operate the arms U and X and the arms W and Z. However, when the input voltage is reduced, the pulse pattern as shown in FIG. 5C and a voltage command pattern are used for the operation.